Happy Earth Day! ( I Killed a Tree)

OK -- maybe I didn't kill it, but it's definitely dead. I've been in denial for a couple weeks. It had the start of little buds and I was hoping it was just a late bloomer. I watered and fertilized and talked to it - but I finally confirmed what I think I knew all along. It's dead. I'm really sad about it. I almost cried.

I planted four Bradford Pear trees four years ago. They were finally getting big and looking like real trees. (The first one in this picture is a Monterey Oak that I planted 2 years ago -- the Pear trees are the ones after that). I had the trees carefully spaced and staggered and now this ruins the whole effect. I am going to plant a new one but it will be a silly little runt compared to the others.

And now I want to know what killed it so it doesn't happen again. Maybe it's on a rock, or had somehow become root-bound, but until recently it was growing just fine. I did some CSIing and noticed several small holes. Maybe some insect killed it??? RUDE.